The present invention relates to a zoom lens which has a high aperture ratio and a high variable magnification ratio, and which is a low cost and appropriate for sill cameras or video cameras.
Conventionally, a means in which a plastic lens is used for a high variable magnification ratio zoom lens, and thereby its cost is reduced, is widely known and many patent applications are made. A zoom lens in which 3 plastic lenses are used in 12 lens elements for cost reduction and which attains a high variable magnification ratio of 10-12 times, is disclosed in Japanese Patent Publication Open to Public Inspection No. 264902/1993. In Japanese Patent Publication Open to Public Inspection No. 106046/1996, a zoom lens is disclosed, in which further cost reduction is promoted, and even when 4 plastic lenses are used in 10 lens elements, the high variable magnification ratio of 12 times is attained. However, in any example, no plastic lens is used in the first lens group, and room to promote further cost reduction is left.
Incidentally, when a plastic lens is used for the first lens, there is a problem of damage caused by an impulse from the outside. As one means to solve the problem, there is a method to equip a protective glass on the object side of the first lens. However, the protective glass contributes to increase a cost, and therefore, it prevents the object of cost reduction. As another means, there is a means to conduct special coating (hereinafter, called hard coat) on the plastic lens surface to protect the surface. However, the hard coat to protect the plastic lens surface can be coated on only material having very good moisture resistance.
There are few types of plastic which can be used as optical materials, and types of high dispersion material are specifically fewer. Therefore, there is almost no room to select the material having very good moisture resistance, and the present status is that it is very difficult to conduct the hard coat on the high dispersion plastic lens. Comparing to this, recently, types of low dispersion plastic are relatively, gradually increasing, and material having highly excellent moisture resistance such as polyolefin, or the like, is put into practical use, thereby, the hard coat can be conducted on the low dispersion plastic lens.
Examples in which plastic lens is used in the first lens group, are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 53718/1982, 13211/1984, and 34882/1994.
However, in the example of Japanese Patent Publication Open to Public Inspection No. 53718/1982, the first lens is a negative plastic lens, and high dispersion material is used, therefore, it is difficult to conduct the hard coat in order to protect the lens surface. Further, an example is disclosed in Japanese Patent Publication Open to Public Inspection No. 13211/1984, in which the composition of the first lens group is sequentially in the order of a positive lens, a negative lens, and a positive lens from the object side, and the first lens is made of glass lens, and the second and third lenses are made of plastic lenses. In such the composition, circumstantial characteristics of the first lens and that of the second lens are very different, and the first lens and the second lens can not be cemented; the decentering sensitivity of the first lens and the second lens becomes high; and disused light tends to totally reflect on the second surface of the first lens, and there is a possibility that ghost is generated. Further, in Japanese Patent Publication Open to Public Inspection No. 34882/1994, a plastic lens is used for only the third lens of the first lens group, therefore, room for further cost reduction is left.
Above-mentioned conventional examples disclosed in Japanese Patent Publication Open to Public Inspection Nos. 264902/1993 and 106046/1996, have 4 lens groups in the order of positive, negative, positive and positive lenses from the object side, and the second lens group has the variable magnification function, and the fourth lens group has a correction function to correct the focal plane movement according to the variable magnification, which are well known as a zoom lens having above functions.
However, in any of conventional examples, more than half of lens components are glass lenses, and therefore, room to promote the more cost reduction is left.